1. Field of the Invention
The invention relates to a process for producing fine monofilaments having improved abrasion resistance from polypropylene having a melt flow index (MFI)230° C./2.16 kg of 2–16 g/10 min, to a monofilament of polypropylene having a melt flow index (MFI)230° C./2.16 kg of 2–16 g/10 min having improved abrasion resistance and a linear density of 5–20 dtex (0.027 mm–0.053 mm) and also to the use thereof.
2. Description of Related Art
Industrial fabrics composed of polypropylene are becoming of increased interest in the automotive industry, in particular because they are lighter and more stable to environmental effects and mechanical stress than other thermoplastic materials. There is a particular demand for fine monofilaments, which permit a further weight reduction. By fine monofilaments are meant monofilaments having a linear density of less than 30 dtex and especially less than 25 dtex.
However, monofilaments composed of polypropylene only have the disadvantage of severe dusting in the weaving operation as a consequence of the low abrasion resistance of pure polypropylene, although other thermoplastics are known to have an abrasion problem too. For instance, EP-A2 0 784 107 mentions melt-spun polyamide, polyester and polypropylene monofils and shows that abrasion-resistant filaments are obtained with 70–99% by weight of fibre-forming polymer and 1–30% by weight of a maleic anhydride modified polyethylene-polypropylene rubber and further additives. However, the examples are limited to nylon 6 and polyethylene terephthalate and to a copolyamide of PA66 and PA6 as fibre-forming polymer. Spinning speeds are not reported. The relatively thick monofilaments exemplified are useful for papermachine wire fabrics and lawn mower wires. The production of relatively fine polypropylene monofilaments is not disclosed.
EP-A-1059370 discloses a method for the production of polypropylene multifilaments for textile purposes. The starting material used is a metallocene-catalysed isotactic polypropylene having a melt flow index of less than 25 g per 10 minutes in order that the desired shrinkage properties may be achieved. Low-shrinkage filaments are preferably produced using polypropylene chips having a high MFI value. The yarns produced are only described in general terms. Monofilaments are not described at all.
EP-A-0028844 describes a textile multifil polypropylene filament yarn. The starting polymer is a polypropylene having a melt flow index between about 20 and 60. The problem of abrasion encountered in the processing of fine monofilaments was evidently not observed under the reported spinning and stretching conditions and in the course of the further processing.